An exposure apparatus known as a stepper is a main production machine for semiconductor fabrication. A stepper is used to image circuit patterns recorded on a mask and print them through a projection lens on a wafer using a step and repeat procedure. Generally, the circuit patterns are stratified into many layers. To retain LSI (Large Scale Integrated) circuit performance, a given layer must be accurately aligned to a previously patterned layer on the wafer. Alignment is frequently accomplished by detecting alignment marks on the wafer. Prior to exposure, the wafer is usually coated with a photoresist-type material using a spin-coating method. In the coating process, the photoresist spreads radially, and often forms an asymmetrical shape around the alignment marks, particularly when such marks are raised or recessed with respect to surrounding, relatively planar areas on the wafer or substrate. The intensity of light reflected from the photoresist is sensitive to the thickness of the photoresist. Therefore, images of the asymmetrically covered alignment marks can be deformed asymmetrically; this produces degradation in the alignment accuracy. However, it is well known that broad-band illumination can reduce this problem. Using broad-band illumination, the intensity of the reflected light is less sensitive to fluctuation of the photoresist thickness, and symmetrical images can be expected in spite of asymmetrical photoresist coverage.
A broad-band light alignment system, which is separated from the projection lens, i.e., a non-TTL (Through-The-Lens) system is described by Nishi in U.S. Pat. No. 4,962,318, issued Oct. 9, 1990. However, there are difficulties with Nishi's system. In practice, the projection lens is mounted using structures of steel, and the lens can move slightly due to environmental factors, such as temperature variations or vibration. A non-TTL system cannot respond to such movement of the projection lens, hence, there are alignment errors. In contrast, a through-the-lens (TTL) alignment system incorporates environmentally induced movements of the projection lens assembly of the stepper automatically, a feature which is essential for accurate alignment. There is, however, a different problem with a TTL alignment system using broad-band light. Normally, the projection lens is designed to have the best performance at a single exposure wavelength (typically, an ultraviolet wavelength). The wavelength of the light used for alignment should be selected at a different wavelength from the exposure light so as not to expose the photoresist. Therefore, TTL systems using broad-band alignment light have difficulty generating high quality images due to the longitudinal and latitudinal or lateral chromatic aberration introduced by the projection lens. To overcome this problem, a chromatic aberration correction system was described by Komoriya et. al. in U.S. Pat. No. 5,094,539, issued Mar. 10, 1992. The Komoriya et. al. correction system consists of more than ten refractive lens elements, which is complicated and expensive to fabricate and to maintain. Yoshitake et. al. describe in JP4-019800, filed in Japan on Feb. 5, 1992, a reduction projection exposure system comprising a reduction projection lens optimized at one wavelength, an illumination apparatus illuminating alignment marks on a substrate through the reduction projection lens with broad-band light, and an alignment mark detection apparatus having an optically diffractive holographic element intended for correcting chromatic aberrations caused in the reduction projection lens during the alignment mark detection. However, the Yoshitake et. al. disclosure fails to provide any lateral chromatic aberration correction, and provides only marginal longitudinal chromatic aberration correction with the holographic diffractive optical element in the alignment apparatus. Furthermore, diffractive elements of the holographic type are generally less effective transmitters of light than diffractive elements of the kinoform type or of the blazed-grating type. Highest possible light transmission is important in an alignment mark detection system.
To counteract the spectral or wavelength dispersion introduced by the projection lens, Komoriya et. al. had to employ many refractive lens elements. The projection lens itself usually consists of more than 10 glass elements, and each glass element introduces dispersion, which causes chromatic aberration. Therefore, using a conventional approach, to correct these aberrations, one essentially needs to use a comparable number of refractive lenses in the alignment system, as disclosed in Komoriya et. al. To reduce the number of elements, at least one element in the correction system must provide a large and negative dispersion value. Glass elements cannot satisfy this requirement. However, a diffractive lens has precisely these characteristics. The dispersion (or Abbe V-number) of a solely diffractive lens element can be -3.45, while that of any refractive glass lens element ranges between 20 and 80. The V-number of a diffractive lens is approximately seven times more dispersive than any known glass and exhibits a negative dispersion, i.e., with a diffractive lens a red ray of light bends more than a blue ray, whereas with a refractive glass lens blue bends more than red. The operation of a diffractive lens relies on interference and diffraction, rather than on refraction of light as in a glass lens. Diffractive lenses include holographic optical elements, blazed gratings and kinoforms. By utilizing a diffractive lens, it is possible to significantly reduce the number of elements necessary for the correction system. This reduction of the number of elements has been disclosed by Yoshitake et. al. in the aforementioned JP4-019800 patent application, although with inadequate correction for chromatic aberrations.